PROJECT SUMMARY/ABSTRACT Over the past few years, increasing evidence has supported the existence of extracellular tau and the concept of transcellular propagation of tau species as a mechanism for the initiation and progression of Alzheimer's disease. In this context, using antibodies to target tau pathology appeared an appropriate approach to clear neurofibrillary tangles in Alzheimer's disease (AD) models. A conspicuous amount of data has been produced by different laboratories showing efficient reduction of tau pathology in transgenic animal models using tau monoclonal antibodies, with a different degree of success according to the epitopes targeted. Nevertheless, the mechanism by which tau antibodies function as treatments for tau pathology is still unclear. To address this point, we will employ engineered antibodies composed of the variable regions of the heavy and light chains joined by a short linker (scFv), preserving the epitope specificity and affinity of the parent antibodies. The work proposed in this application is designed to use tau scFvs carried by viral vectors (AAV) in order to specifically target neurons or astrocytes in vitro and in vivo. We will clone into viral vectors the scFvs corresponding to different tau antibodies previously tested in passive immunization experiments. These engineered tools will be used first in vitro in order to address the question whether the most efficient strategy to reduce tau phosphorylation/aggregation will be to target tau acting from the extracellular milieu or from the intraneuronal compartment. Furthermore, we will be testing if by providing a chronic release of recombinant antibodies directly in the mice brain parenchyma, either by intraneuronal production or by extracellular exposure, we can define the mechanism by which these antibodies exert their therapeutic effects as well as developing an efficient strategy to reduce tau aggregates in transgenic animal models and potentially in humans.